A generic method and a generic hydraulic apparatus are known from DE 196 80 006 C1. It includes a pump, which has a volumetric delivery characteristic resulting in cyclic pulsing, and a servo motor having a plurality of poles resulting in cyclic pulsing. The pressure to the consumer is applied by way of a pressure sensor of a pressure regulator that adjusts the servo motor with the aid of a predetermined or predeterminable pressure profile. Moreover, the rotary angle of the pump and/or the servo motor is detected by way of a rotary encoder.
An apparatus and a method for minimising cyclic pulsing are known from DE 103 16 946 A1. The cyclic pulsing is detected by a sensor, and the measured values are processed in an open-loop/closed-loop control device for triggering an actuator which generates pulses that are at least approximately of the opposite phase to the detected pulsing.
An electrohydraulic control arrangement which can also be used in a plastics injection moulding machine is known from DE 10 2008 019 501 A1. An adjustable fluid pump is connected to a variable-speed electric drive. Pressure and/or volumetric flow are detected and adjusted, by way of a pressure/quantity regulator, to a setpoint value in that a rotational speed setpoint value is provided by means of a variable-frequency drive.
EP 0 431 150 B1 discloses using a brushless dc motor for servo drives, and in so doing minimising cogging, in that the servo motor is operated with a trapezoidal trigger signal and the angular position of the rotor is detected. The trigger signal and the angular position are superimposed such that cogging is reduced. Cogging arises because, in electric motors, the rotor is guided alternately over magnetic and non-magnetic material, with the result that a corresponding detent torque may occur. This generates a sinusoidal current whereof commutation from one winding or magnet to the next is the primary cause of an eccentric running characteristic.
EP 0 649 722 B1 discloses a pressure regulator for a plastics injection moulding machine in which a regulating pump follows the demand from the respective hydraulic consumer, by way of pressure regulator. In addition, the regulating pump is driven by means of a three-phase motor whereof the rotational speed is regulated by a variable-frequency drive such that the pump output of the regulating pump is regulated at the regulating member of the pump in order to keep it below its maximum output, determined by the variable-frequency drive. This makes energy-saving operation possible.
A common feature of the prior art mentioned is that in principle a hydraulic drive circuit is formed by a pump with a servo motor coupled thereto. A pump of this kind may be a fixed displacement pump or a variable displacement pump such as an axial piston pump or a radial pump. Typically, although pumps of this kind convey fluid continuously, this is performed using elements that operate in pulsing manner. Thus, for example, a fixed displacement pump such as an internal gear pump has a certain number of conveying teeth, determined by its construction. In the prior art, hydraulic drive systems of this kind are termed a “servo pump” and used for plastics injection moulding machines as the central drive hydraulics if the machine axes perform serial sequences and are to run to the optimum from an energy point of view. As a result of the large range of rotational speeds at which both the pumps and the servo motors may be operated, it is frequently possible for very large volume outputs to be generated at high speeds with relatively small volumes conveyed by the pump on each revolution. Should the hydraulic consumer need a quasi-static pressure to be regulated, the latter may be regulated by means of a pressure sensor mounted in the hydraulic line to the consumer and corresponding rotational speed regulation of the servo motor with associated open-loop control at a servo motor speed of close to zero, since there are only minimal internal leaks in the pump and consumer system. However, it is specifically in this condition that pulsing of the conveying flow of the pump becomes apparent.
Commercially available pumps, such as, in the case of a fixed displacement pump, a gear pump but also a piston pump, do not have a volumetric flow of fixed speed, in relation to a revolution of the pump, but have a pulse of a certain quantity. This quantity pulse results from the internal mechanical symmetry of the pump construction, such as the number of conveying teeth. In particular in the case of very low rotational speeds and high pressures, the result is thus considerable pressure pulsing, which—for example in the holding pressure phase of an injection moulding machine—has a relatively great effect on the process.
Similar pressure pulsing may also be caused by the structure of the servo motor. The rotor of the servo motor moves within the stator as a result of an electromagnetic field which is generated by applying current and which is generated by commutation of the poles. However, the application of a sinusoidal current also results in a detent torque, so-called cogging, which, in particular at low rotational speed, may also result in pulsing which is attributable to the arrangement of the magnetic and non-magnetic elements within the stator.
Pressure pulsing of this kind could be adjusted in the same way as that sought by the pressure regulator, by regulating the predetermined or predeterminable process pressure. However, the consequence of this is that a very rapid and complex regulator would have to be created, since the pulsing that occurs is affected by the greatest possible variety of system properties, such as the total volumes of oil in the consumer line downstream of the pump, the oil temperature, or the axial position of the consumer. Moreover, the motor and pump have to be coupled very rigidly from a control engineering point of view in order to bring about a pressure profile that is acceptable from a process engineering point of view. The consequence of this is a very high electrical excitation or the application of a very large current, so it is disadvantageous from an energy perspective.